I couldn't say whether clipless pedals actually increase efficiency much, but clipping in makes you feel "one with the bike" - an awesome feeling! Before using them I did not feel I had a secure connection with the bike, so more effort was focused on maintaining that connection. Clipless allows me to simply focus on the peddling cadence.

I have only fallen once (so far) - the second ride with clipless, I pulled up to an intersection, hoping to ride through it, but there was traffic coming, so I had to stop. I was going to balance the bike for the few seconds required, but then changed my mind & went to take my foot off the pedal, realized it was clipped in ... & toppled into a large puddle. It was embarrassing.